Presses for the hot pressing of flat objects, in the form of platen presses, are provided in a variety of configurations and with one or more heatable press plates or platens. Reference may be had to the following United States patents and the literature cited in the files thereof or referred to in the respective texts: U.S. Pat. No. 3,687,788, No. 3,619,322, No. 3,428,505, No. 3,517,610, No. 3,565,725, No. 3,914,079 and No. 3,989,581.
From the foregoing, it will be apparent that heatable press plates can be used in so-called single-level or single-stage presses, in which the article to be pressed is received between a bed plate and a head plate, at least one of which is heated on a tray, on a conveyor or directly, with either the head plate or the bed plate, or both being movable to press the article between them.
Double-level or two-stage presses can also be provided with two articles received between an intermediate press plate and the bed plate and head plate respectively.
Heatable plates are also used in multistage or multilevel presses in which, between the head plate and the bed plate there are received a large number of press plates with or without a simultaneous closing mechanism on the bed plate and each of the intermediate plates, a flat article to be pressed can be received.
In all of the aforedescribed presses, at least one and preferably both of the plates between which an article is sandwiched, can be heated by passing a heating fluid, e.g. superheated steam, through passages in the plate between an inlet and an outlet.
The presses can be used for the production of pressed-board in which a flat collection of fibers or other particles of wood or other cellulosic material, with or without binder, is hot pressed into a coherent slab. The collection of particles may be more or less coherent and may be in the form of a mat and the product may be used for a variety of purposes, depending on its composition and density, the latter being in part a function of the heating and pressing operation. For example, the pressed-board may be a low density highly porous, light weightparticleboard which can be used primarily for insulation and for structural purpose in which the board does not constitute a load-bearing element.
The systems can be used also for the production of high-density, high-strength particleboard which is not only self-supportive, but is load-bearing and weather-resistant. Naturally, the entire range of particleboard applications between those described can be provided as well.
The presses can also be used for finishing particleboard or for the fabrication of laminated board by using the hot pressing action to bind one or more surface-finishing or decorative layers to a core or substrate, e.g. of particleboard, by lamination techniques.
Presses of the type described have also been used to form from superposed layers, e.g. of wood or combination of wood and synthetic resin materials and for the production of synthetic resin or rubber boards, slabs or belts (e.g. conveyor belts).
The heating plates or platens of the presses can have the heating-fluid passages in the form of mutually parallel bores of generally circular cross section. The bores may be provided in pairs separated by regions of small wall thickness with the pairs of bores being separated by walls of larger thickness so that, in cross section, the partitions between the bores have the appearance of hyperboloids.
With presses in which heating and cooling alternate and in which the cooling is effected by forcing water through the passages previously traversed by superheated steam, it is found that a homogeneous temperature distribution is difficult, if not impossible to maintain in conventional press plates of the aforedescribed type.
The failure to maintain a homogeneous temperature distribution seriously affects the quality of the products made e.g. can lead to warping, products with lack of isotropy in various portions, various surface-finish defects which cannot be removed by subsequent treatments or which can only be removed by expensive and time-consuming procedures.
In the prior art systems of the aforedescribed type, the partitions or walls between the bores tended to terminate immediately at the points at which the bores opened into the chambers at which the fluid was deflected from flow in one direction to flow in the opposite direction, i.e. from one pair of bores into the next pair of bores. In other words, the thick partition between pairs of bores and the thin partitions between the bores of each pair terminated in the same plane perpendicular to the plane of the plate. Furthermore, all of the edges contacted by the fluid in the region of each direction-changing chamber were usually rounded.